This invention relates to a process and apparatus for induction hardening hot work die steels and mandrel bars for tube piercing mills made using that process and apparatus. More particularly, this invention relates to induction hardening of through hardened chromium-molybdenum hot work die steels (Class 520, which includes AISI types H-10, H-11, H-12, and H-13) to achieve a tough center and high case hardness and an apparatus for induction hardening long workpieces with a minimum of distortion.
The steels in Class 520 are the most widely used of all hot work die steels because of their toughness and shock resistance. Typical applications for Class 520 steels are die-casting dies, forging dies, punches, piercers and mandrels for hot work, hot extrusion tooling, shear blades for hot work, and all types of dies for hot work that involves shock. (For a description of the chemistry and physical properties of Class 520 steels, see Roberts and Cary, Tool Steels, American Society for Metals, Metals Park, Ohio 44073, at pp. 578-79 (4th edition, 1980).)
Case hardening of products made from chromium-molybdenum hot work die steels, such as mandrel bars for tube piercing mills, generally is accomplished by nitriding or chrome plating. These techniques produce a thin case. With nitriding, for example, the depth of hardness is approximately 0.025 inch (0.635 mm.). Chrome plating produces an even thinner case than nitriding. Such thin cases wear out quickly, thus shortening the life of workpieces case hardened by these techniques. In addition, there is a potential problem of hydrogen embrittlement in chrome plating and a white layer effect in nitriding.
Case hardening by induction heating allows a greater depth of hardness than nitriding or chrome plating. Induction heating is performed by passing an alternating current through a scanning induction coil surrounding the material to be heated. The rapidly alternating electrical field, in which the material to be heated is held, causes the steel to heat very rapidly, due to eddy currents and hysteresis. Depth of hardness is controlled by varying the power level and frequency of the alternating current and travel speed of the inductor coil.
Induction heating has been used to case harden heat treated carbon and low alloy grades of steel which are not through hardening grades. Induction heating has not been used to case harden through hardened high alloy steels such as those in Class 520. This is most likely because one skilled in the art would expect such induction hardening of high alloy steels to result in surface peeling and cracking because of high stresses associated with high alloy steel.
One problem with induction hardening, however, is that it causes distortion in the workpiece as hardening progresses. The tendency to distort increases as the length that has been induction hardened increases. For example, it has been found that there is as much as 1/4 inch (6.35 mm.) bow in a workpiece after an inductor coil travel distance of approximately 10 feet (3048 mm.). Because of distortion, the workpiece moves closer to the inductor coil in one direction and farther from the coil in another direction. With the resulting uneven heating, the severity of the distortion quickly multiplies. Therefore, it will be realized that a uniform gap or distance between the workpiece and inductor coil is very important in the prevention of the multiple distortion effect.
In conventional induction hardenening equipment, the scanning inductor coil is fixed so that the inductor coil can move only in a forward or backward direction along the length of the workpiece. Because the usual gap between the inductor coil and workpiece is very small, about 3/16-1/4 inch (4.76-6.35 mm.), eventual workpiece distortion has been observed to cause the inductor coil to collide with the workpiece causing arcing and cessation of scanning.
One possible way to lessen the multiple distortion effect and to prevent collisions between the coil and workpiece resulting from distortion is to increase the gap or distance between the inductor coil and the workpiece. However, this is impractical because the bigger the gap, the less efficient the heating process. Another possible way to lessen distortion is to scan in increments, letting one small area harden before proceeding to the next area. However, such interrupted or non-continuous scanning results in "soft spots," i.e., a non-uniform surface hardness and case depth, and possible surface cracking.
Distortion can be corrected after induction hardening by forming the workpiece with an extra material or "stock" above the specified diameter which is machined away where necessary after induction hardening. However, the extra machining step is time consuming and costly, and results in a loss of case depth.